The discoveries of endogenous morphine-like peptides have generated intense interest in their structure-activity relationships. All correspond to portions of the structure of .beta.-lipotropin and possess biological activity to varying degress in in vitro test systems. In contrast, the in vivo analgesic activity of enkephalin injected intracranially appears to be of short duration, while that of .beta.-endorphin is long lasting. Thus far only .beta.-endorphin displays in vivo analgesic activity when injected into the peripheral system. One approach toward understanding the properties of .beta.-endorphin which make it similar to, yet dissimilar from, all the other opioid peptides is through total synthesis of its analogs. The syntheses of camel and human .beta.-endorphins in good yields established conditions which provide facile access to these compounds. Reports on the enzymatic cleavage of the Tyr-Gly bond in enkephalin which can account for the transiency of its analgesic activity led to the development of the analogs [D-Ala.sup.2 ]-enkephalin (Hambrook et al., Nature (Lond.) 258, 577 (1976) and its corresponding amide (Pert et al., Science 194, 330 (1976) which exhibit long-lasting analgesia when injected centrally.